<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .x {
            border: solid 1px red;
            width: 100px;
            height: 100px;
            position: relative;
            background-color: transparent;
        }

        .x:before {
            position: absolute;
            top: 0px;
            right: 0;
            left: 0;
            bottom: 0;
            border-bottom: 100px solid red;
            border-left: 100px solid transparent;
            content: "";
        }

        .x:after {
            position: absolute;
            left: 1px;
            right: 0;
            top: 1px;
            bottom: 0;
            border-bottom: 99px solid white;
            border-left: 99px solid transparent;
            content: "";
        }

        .f {
            border: solid 1px red;
            width: 100px;
            height: 100px;
            position: relative;
            background-color: transparent;
        }

        .f:before {
            position: absolute;
            top: 0px;
            right: 0;
            left: 0;
            bottom: 0;
            border-bottom: 100px solid red;
            border-right: 100px solid transparent;
            content: "";
        }

        .f:after {
            position: absolute;
            left: 0;
            right: 1px;
            top: 1px;
            bottom: 0;
            border-bottom: 99px solid white;
            border-right: 99px solid transparent;
            content: "";
        }
    </style>
</head>

<body>
    <div class="x"></div>
    <div class="f"></div>
</body>

</html>